cmd/compile: refactor symbol sorting logic

This used to be duplicated in methcmp and siglt, because Sig used its
own representation for Syms. Instead, just use Syms, and add a
(*Sym).Less method that both methcmp and siglt can use.

Also, prune some impossible cases purportedly related to blank
methods: the Go spec disallows blank methods in interface method sets,
and addmethod drops blank methods without actually recording them in
the type's method set.

Passes toolstash-check.

Updates #24693.

Change-Id: I24e981659b68504d71518160486989a82505f513
Reviewed-on: https://go-review.googlesource.com/105936
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 26 insertions, 0 deletions

diff --git a/src/cmd/compile/internal/types/sym.go b/src/cmd/compile/internal/types/sym.go
index e9b454d83a..fe6ddbf5a2 100644
--- a/src/cmd/compile/internal/types/sym.go
+++ b/src/cmd/compile/internal/types/sym.go
@@ -77,6 +77,32 @@ func (sym *Sym) Linksym() *obj.LSym {
 	return Ctxt.Lookup(sym.LinksymName())
 }
 
+// Less reports whether symbol a is ordered before symbol b.
+//
+// Symbols are ordered exported before non-exported, then by name, and
+// finally (for non-exported symbols) by package path.
+func (a *Sym) Less(b *Sym) bool {
+	if a == b {
+		return false
+	}
+
+	// Exported symbols before non-exported.
+	ea := IsExported(a.Name)
+	eb := IsExported(b.Name)
+	if ea != eb {
+		return ea
+	}
+
+	// Order by name and then (for non-exported names) by package.
+	if a.Name != b.Name {
+		return a.Name < b.Name
+	}
+	if !ea {
+		return a.Pkg.Path < b.Pkg.Path
+	}
+	return false
+}
+
 // IsExported reports whether name is an exported Go symbol (that is,
 // whether it begins with an upper-case letter).
 func IsExported(name string) bool {